1. Field of the Invention
The present invention relates to a magneto-optic optical device, such as a variable optical attenuator, an optical modulator or an optical switch, used for an optical communication system.
2. Description of the Related Art
As a variable optical attenuator, there is known a so-called magneto-optic variable optical attenuator which changes a Faraday rotation angle by the intensity of an applied magnetic field to control light attenuation. The magneto-optic variable optical attenuator has merits that since there is no mechanical movable part, its reliability is high and miniaturization is easy. The magneto-optic variable optical attenuator includes a magnetooptical crystal and an electromagnet for applying a magnetic field to the magnetooptical crystal. The quantity of a current flowing to the electromagnet is changed to control the intensity of the magnetic field applied to the magnetooptical crystal, so that the intensity of magnetization of the magnetooptical crystal is changed and the Faraday rotation angle can be controlled.
A method of controlling a magnetic field applied to a magnetooptical crystal is disclosed in, for example, patent document 1 (Japanese Patent No. 2815509). This magnetic field control method will be described with reference to FIGS. 13A and 13B. FIG. 13A shows a variable optical attenuator, and the variable optical attenuator includes a Faraday rotator (magnetooptical crystal) 113 and a polarizer 112. Besides, the variable optical attenuator includes a permanent magnet 114 and an electromagnet 115 for applying magnetic fields to the Faraday rotator 113 in directions orthogonal to each other, and a variable current source 116 for feeding a driving current to the electromagnet 15.
The direction of the magnetic field applied to the Faraday rotator 113 by the permanent magnet 114 is parallel to the transmission direction of a light beam 117 in the Faraday rotator 113. The direction of the magnetic field applied to the Faraday rotator 113 by the electromagnet 115 is vertical to the application direction of the magnetic field of the permanent magnet 114 and the transmission direction of the light beam 117 in the Faraday rotator 113.
In FIG. 13B, each of arrows 102 and 105 is a vector indicating the direction of magnetization and its magnitude in the Faraday rotator 113, and each of arrows 101, 104 and 103 is a vector indicating the direction and magnitude of the application magnetic field applied from the outside. A Z direction in the drawing is the propagation direction of light in the Faraday rotator 113, and an X direction is orthogonal to the Z direction. The Faraday rotator 113 is put in a state of the saturation magnetization 102 by the vertical magnetic field 101 of the external permanent magnet 114. Next, when the horizontal magnetic field 103 of the electromagnet 115 is applied, the external magnetic field becomes the combined magnetic field 104, and the Faraday rotator 113 is put in a state of the magnetization 105. The magnitude of the magnetization 105 is equal to the magnitude of the saturation magnetization 102, and accordingly, the Faraday rotator 113 is in the state of the saturation magnetization.
As stated above, the vertical magnetic field is previously applied to the Faraday rotator 113 by the permanent magnet 114 to put the Faraday rotator 113 in the state of the saturation magnetization, and the horizontal magnetic field is further applied by the electromagnet 115 disposed in the in-plane direction of the Faraday rotator 113. Then, the direction of the magnetization of the Faraday rotator 113 is rotated by the combined magnetic field 104 of the two magnetic fields from the magnetization 102 to the magnetization 105 by an angle xcex8, and the magnitude of a magnetization component 106 in the Z direction is controlled. The Faraday rotation angle is changed in dependence on the magnitude of the magnetization component 106. This method has features that since the Faraday rotator 113 is always used in the saturation magnetization region, hysteresis does not occur, and the Faraday rotation angle can be changed with excellent reproducibility.
Incidentally, the following documents are cited for reference.
[Patent document 1]
Japanese Patent No. 2815509
Counterpart patent: U.S. Pat. No. 5,889,609
[Patent document 2]
JP-A-7-1042251
Counterpart patent: U.S. Pat. No. 5,535,046
[Patent Document 3]
U.S. Pat. No. 5,477,376
[Patent Document 4]
U.S. Pat. No. 6,198,567
However, in the magnetic field application method disclosed in the above-mentioned patent document 1, the magnetization is uniformly rotated in the state where the vertical direction magnetic field by the permanent magnet 114 is applied. It is necessary to enhance the in-plane direction magnetic field applied by the electromagnet 115, and the electromagnet 115 must be made large or a large current must be made to flow. Therefore, there is a problem that miniaturization and reduction in consumed electric power are difficult.
An object of the invention is to provide a magneto-optic optical device, such as a variable optical attenuator, an optical modulator or an optical switch, which is small, has a low power consumption, and has a high speed.
The above object is achieved by a magneto-optic optical device characterized by comprising at least one magnetooptical crystal, a magnetic field application mechanism for applying to the magnetooptical crystal a magnetic field component in a direction vertical to a light entrance/exit plane, and at least one electromagnet for changing a position where the magnetic field component applied to the magnetooptical crystal becomes 0.
The above magneto-optic optical device of the invention is characterized in that the magnetic field application mechanism includes at least one permanent magnet. Besides, a magnitude of the magnetic field component is monotonously changed in a specified direction in the light entrance/exit plane.
The above magneto-optic optical device of the invention is characterized in that the magnetooptical crystal includes a magnetic domain A constituted by a magnetization in a direction vertical to the light entrance/exit plane, and a magnetic domain B constituted by a magnetization in an opposite direction to the magnetization direction of the magnetic domain A.
The above magneto-optic optical device of the invention is characterized in that the magnetic field generated by the electromagnet is changed to form a state where only the magnetic domain A exists in a light transmission region of the magnetooptical crystal and a state where both the magnetic domain A and the magnetic domain B are contained, and a transmitted light intensity is continuously changed.
The above magneto-optic optical device of the invention is characterized in that a state where only the magnetic domain B exists is formed. Besides, a boundary between the magnetic domain A and the magnetic domain B is almost linear.
The above magneto-optic optical device of the invention is characterized in that a saturation Faraday rotation angle of the magnetooptical crystal is about 45xc2x0, and the magneto-optic optical device includes a polarizer disposed at one side of the magnetooptical crystal, and an analyzer disposed at the opposite side of the magnetooptical crystal.
Besides, the above magneto-optic optical device of the invention is characterized in that a saturation Faraday rotation angle of the magnetooptical crystal is about 90xc2x0, and the magneto-optic optical device includes a polarizer disposed at one side of the magnetooptical crystal, and an analyzer disposed at the opposite side of the magnetooptical crystal.
The above magneto-optic optical device of the invention is characterized in that a saturation Faraday rotation angle of the magnetooptical crystal is about 45xc2x0, and the magneto-optic optical device includes a polarizer disposed at one side of the magnetooptical crystal, and a reflecting film disposed at the opposite side of the magnetooptical crystal.
The magneto-optic optical device of the invention is characterized in that the magneto-optic optical device is a variable optical attenuator for variably controlling an attenuation by changing a current applied to the electromagnet.
The magneto-optic optical device of the invention is characterized in that the magneto-optic optical device is an optical modulator for modulating a transmitted light intensity by modulating a current applied to the electromagnet. Besides, the magneto-optic optical device is an optical switch.
According to the invention, the magnetization is not uniformly rotated, and the magnetic domain structure in the light transmission region is changed, and therefore, it is possible to realize the magneto-optic optical device, such as a variable optical attenuator, in which a small electromagnet can be used, or a current flowing to the electromagnet can be made a low current.